1. Field of the Invention
The invention relates to an optical scanning system using a vibratory or oscillatory diffractive element made by micromachined or Micro-Electro-Mechanical (MEM) technology.
2. Discussion of the Related Art
Optical scanning devices have numerous applications in the following fields: manufacturing, medicine, military hardware, information systems and communications. These applications include but not limited to optical switching, biomedical imaging (optical coherence tomography, scanning laser confocal microscopy etc.), laser printing, spectrometer, microdisplays, image projection and acquisition, laser materials processing (marking, drilling, coding trimming etc.), and stereolithography.
The primary function of an optical scanner device is to steer an electromagnetic beam, typically visible light (wavelength from 400 nm to 700 nm) along a single or multiple axes. For all the application fields, the basic requirements for a high-end good performance scanner are a large scan angle and a high operating frequency, resonant or non-resonant. There are many scanning methods to achieve these, among them are holographic, polygonal, galvanometric, resonant, acousto-optic, and electro-optic techniques. The electro-optic and acousto-optic scanners have a high operating frequency in the MHz range but with an inherent small scan angle of about 4 degrees. The other scanners, which are mainly mechanical in nature, have a larger scan angle of about 30 degrees but are limited to a low operating frequency in the range of 200 to 10,000 Hz.
All of the above existing optical scanners until now have been conventionally manufactured. Recently a new group of scanners have emerged that are fabricated using the microfabrication techniques similar to the microelectronics circuit. These micromachined optical scanners have attracted much attention because of their outstanding advantages compared to conventional macro-scanners. These advantages include having a small mass leading to a high resonant frequency, compact small size, low power consumption, ease of integrating with CMOS circuitry, light weight and low per unit cost due to batch fabrication. These benefits will not only provide performance enhancements such as smaller, high-speed, and lower-cost to existing applications but also form the technological basis for a wide range of new applications in raster-scanning retinal projections and compact high-speed fiber optic components.
The related art microscanners differ in their mechanical structure (topology), operating principle, and actuation mechanism. The related art microscanners include, for example, electrostatic, electromagnetic, electrothermal, piezoelectric, and magnetostrictive types. However, all share a common scanning technology platform that is based on micromirror reflection technology and are actuated out-of-plane. One of the critical drawbacks of this micromirror reflection technology is large dynamic mirror deformation (greater than λ/8 mechanical). This phenomenon is due to the fabrication-limited thickness and high out-of-plane acceleration forces, which cause the mirror plate to deform dynamically during scanning, thus adversely affecting the optical resolution.
The related art micromirror reflection technology can be represented in a generic schematic, as shown in FIG. 3. The micromirror 60 can be made to rotate about an axis defined by two torsional beams 70 through the driving actuator 80. One of the most prominent problems of the scanning micromirror entails the dynamic deformation of the mirror plate. Micromachined mirrors are normally much thinner than conventional macro-scanning mirrors, therefore, at high scanning frequencies, micromachined mirrors deform dynamically, which introduces time-dependent optical aberrations and severely limits the optical resolution.
The optical resolution is defined as the ratio of the mirror scan angle and optical beam divergence (or as the ratio of the length of scan line to the spot size). There are many factors that will affect the optical resolution of a micromachined scanner, for example, beam diffraction, static and dynamic deformation of the micromirror. The static deformation of a micromirror, which is normally caused by the residual stress of the mirror material, can be minimized to a significant degree by introducing mirror curvature compensation optics to the scanner. However, the reduction of scanning beam divergence caused by beam diffraction and micromirror's dynamic deformation is still a big challenge for high-speed scanning micromirror developers. Both diffraction and dynamic deformation are dependent on the mirror size; increasing the mirror size decreases diffraction, while simultaneously increasing the dynamic deformation. Some researchers have used a thick micromirror to achieve a dynamic deformation within the Rayleigh limit (λ/8 mechanical). However, increasing the mirror thickness also increases the mass, and in order to maintain a high scanning frequency, the stiffness of the mirror suspension has to be increased. This results in a very high driving voltage and considerable power consumption.
The only current exception to the micromirror reflection technology that exists in the literature is a diffraction grating scanner using a polysilicon micromotor (as shown in FIG. 4), which is a direct miniaturized version of rotating holographic laser beam scanner. However, this miniaturized scanner has some critical drawbacks. Firstly, the scanning rate is severely limited to a few hundred Hz since the polysilicon micromotor shown in FIG. 4 lacks the potential to spin at a high speed. Secondly, the driving voltage required for motion to be continuous is very large, typically hundreds of volts. Thirdly, the scanner is not free from wear and tear especially when high spinning speed is required, thus severely limiting the lifetime of the scanner.
As discussed above, the design of existing microscanners based on micromirrors and micromotors has been mostly focused on the use of existing fabrication technologies, device designs and architecture without giving sufficient attention to the optical performance of these devices, particularly their performance under dynamic conditions. These limitations of the related art micromachined scanners prevent their successful use in many applications, hence creating a technological trade-off in deriving some obvious advantages of miniaturization but without the accompanying performance benefits of high scanning resolution.
The disadvantages of the conventional micromirror scanners include dynamic deformation: the inability to provide for an optically flat mirror surfaces during scanning such that reflective surfaces are free of phase distortion which will keep the system in optical focus. An additional disadvantage of the conventional micromirror scanners includes high power: the inability to reduce the actuation voltages and operate linearly for large angle of scan.
The disadvantages of micromotor scanners include low scanning rate, which is the inability to provide for a high scan frequency, in the order of tens of kHz. An additional disadvantage is high power consumption arising from the inability to reduce actuation voltages. A further disadvantage arises from a reduced lifetime arising from the inability to operate for an extended period of time due to wear and tear.
As has been shown, there are serious disadvantages that hamper the wide application of conventional vibratory or oscillatory micromirror and micromotor scanners.